The real question (there should be an answer somewhere tho) is "who is already in there"?
as far as I know (i.e. not much) with the original game's roster and the DLC, including the facebook overeem DLC, we'll pretty much have everyone, or close enough.

I'd do a DLC with the first UFC fighters, such as the Sumo Teila Tuli or Gerard Gordeau, 1 hand gloved Art Jimmerson.
the DLC with the UFC 1 arena and commentary, the old days looks.

A DLC with a fighter's career starting at UFC 1 with a scenario following the UFC's history, like at some point the fighter would be able to choose to go to Pride (I think that's already in the story mode), Like I think they did with the last Fight night, the dark ages of the UFC where the fighter would have to do illegal/backyards fights for a while, before the UFC comes back under Zuffa and at full force. With all the consequences, unable to pay for "premium" training and whatnot, maybe go to prison and fight there, bare knuckles fighting etc...

They really missed the boat on that one. That would have been brilliant. They could have avoided all the licensing expenses AND make the fans happy. Ah well, maybe in a couple of more years. The roster is a lot more complete with the DLC.

Think I will use a real UFC fighter in career mode at first since it's the time the franchise has allowed us to do that. Then i'll make a CB Dollaway CAF and have him smoke dudes the way he should be doing in real life

I've got the game, it's a lot of fun but Renan Barao is 29 years old.. that's one mistake i've noticed so far. I don't think anyone has mentioned this but it would be cool if they did a Strikeforce HW DLC Pack, Bigfoot, Werdum, Cormier, Barnett, Lavar Johnson and Shane Del Rosario.

AlexanDER Gustafson, Weidman and Ferguson would be nice. Don't care much for the others though

Regardless of whether you like them or not there's no doubting they are both going to be a force in their respective divisions, if Joe Daddy is on this game there's no reason Edson, Erick, Gustaffson, Weidman, Ferguson, Ellenberger, Story, Brenneman, Hendricks and others shouldn't be.

I have the DLC pack as of about 10 minutes ago, Overeem is a beast, i'm doing title mode with him on advanced difficulty and nobody is even lasting 2 minutes.